Testing Resistance Welding Transformers
The tests described below utilize an ohmmeter. A regular unit will pick up most shorts,
but a mega-ohm meter, if available, will work better. Later steps also ask you to use a
fused 110 VAC power cord. Make sure the cord is fused with a ~5 amp fuse. As always,
exercise due care and caution when working around live electricity.
1. Make sure that the power to the welder is disconnected and the power is lockedout according to plant approved lockout-tagout procedures.
2. Disconnect the line leads running from the transformer or tap switches to the
control.
3. If the transformer has a tap switch connected to it, make sure that it is on a tap and
not in the “off” position.
a) Connect the ohmmeter across the line leads that you disconnected from
the control. You should read zero ohms, or “continuity”, through the
primary of the transformer.
b) Next connect the ohmmeter between either line lead and the secondary of
the transformer. You should read infinite ohms, or “no continuity”.
c) Now connect the ohmmeter between either line lead and the ground (or
frame) of the transformer. You should read infinite ohms, or “no
continuity”.
4. Turn all tap switches to their highest setting.
5. Make sure that the weld tips, or secondary of the transformer is an open, not
complete, circuit. This can be accomplished by placing a piece of rigid insulation
or an old credit card between the tips.
Transformer Testing, page 2
6. Connect the fused 110 VAC cord across the two line leads. Note: If the windings
of the transformer are bad, you will probably blow the fuse in the 110V cord.
a) Check the Secondary output voltage of the transformer with a voltmeter.
This measurement should be made right at the transformer, and not at the
tips. Bad connections in the secondary loop could cause a larger voltage
drop across them. Also make sure that the secondary loop is still open
(above) or the transformer will be under a load.
b) If your welder has a 220 VAC supply, you should read about ½ of the
rated maximum secondary voltage.
c) If your welder has a 440 VAC supply, you should read about ¼ of the
maximum rated secondary voltage.
d) If the voltage reading is close, your transformer is probably good.
7. If you have a clamp type ammeter, you can check the primary current draw of the
110V line. It should be no more than 1 to 2 amps for most transformers.
Transformer Testing, page 3
Frequently Asked Transformer Questions
Question 1: What information is required to size a resistance-welding transformer?
Answer: The information needed is weld current, weld time, production rate
(welds/min.), primary voltage, primary frequency, and a sketch of the secondary circuit
(including cable/shunt/bus bar lengths, cross sections and orientation). Additional
information may be required for certain applications.
Question 2: What is the formula to calculate the effective continuous primary current for
a transformer rated at a 50% duty cycle?
Answer: (KVA50% x 1000) / (Primary Voltage *1.41) This value is also known as
ECTC (Effective Continuous Thermal Current)
Question 3: What causes a resistance-welding transformer to growl or grunt?
Answer: Very likely the cause of the problem is saturation of the transformer core.
Transformer core saturation causes half-cycle firing or imbalanced firing. Viewing the
primary current waveform is the most reliable method to identify the problem. More
investigation may be required to isolate the problem.
Question 4: What is the recommended mixture for the liquid coolant of a welding
transformer?
Answer: The purpose of the coolant is to reduce heat developed by the transformer.
Chemicals may be added to the coolant to protect against algae, corrosion or mineral
deposits. The coolant must be compatible with plumbing components and cannot inhibit
the ability to remove heat. Just like adding antifreeze to your car’s engine, it is best to
minimize the amount in a transformer as well due to poor heat dispersion. RWMA
Bulletin 14 provides guidelines for coolant mixtures.
Question 5: When should I use a ground reactor or a ground strap on the secondary of a
transformer?
Answer: ANSI/AWS Bulletin Z49.1 states, “The secondary of a transformer shall be
grounded by one of two methods or in the case of non-portable machines required the use
of an isolation contactor”. The two methods listed in the bulletin are the permanent
grounding of the welding secondary circuit or the use of a ground reactor. The choice of
using a permanent ground or a ground reactor is an issue of the weld process and the
machine design. If multiple transformers make contact to a common point, a ground
reactor can prevent secondary circulating currents. There are many other situations that a
ground reactor can eliminate circulating currents and should be should be evaluated in the
machine design. Both of these methods are safe and meet the requirements of Z49.1.
Transformer Testing, page 4
Question 6: What is the accuracy of the transformer turns ratio?
Answer: The transformer turns ratio is very accurate. It is controlled by the transformer
manufacturer and is tested several times in the quality control process during
manufacturing. Often the turns ratio is used in a weld control to calculate the secondary
current when the control is monitoring the primary current. It is common practice to
adjust this ratio while calibrating the weld control with an external current meter. The
true turns ratio is the ratio of turns of wire on a primary coil to the number of turns on a
secondary coil. This ratio is a physical quantity and can only be changed by selecting a
different turn on the primary coil through a tap switch or some other hard wired
connection.
Question 7: What is the difference between the terms Inverter DC, High Freq DC
(HFDC) and
Mid Freq DC (MFDC)?
Answer: All three terms refer to the same thing and may be used interchangeably. The
recognized resistance-welding industry term is Inverter DC. Inverter DC transformers
operate at a higher frequency than 60 Hz and have diodes on the output side of the
transformer. The operating range of Inverter DC transformers is between 1000 and 1200
Hz.
Question 8: What is the relationship between KVA and output current?
Answer: A transformer’s KVA rating is a thermal rating and deals with how many welds
per minute it can make and not the weld current it can produce. A higher KVA rating
does not automatically mean it is capable of more welding current. The common
misconception is that resistance-welding transformers are current sources. These types of
transformers are instead voltage sources.